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id="post-meta"><div class="meta-firstline"><span class="post-meta-date"><svg class="meta_icon post-meta-icon" style="width:30px;height:30px;position:relative;top:10px"><use xlink:href="#icon-rili"></use></svg><span class="post-meta-label">发表于 </span><time class="post-meta-date-created" datetime="2022-09-19T06:16:49.000Z" title="发表于 2022-09-19 14:16:49">2022-09-19</time><span class="post-meta-separator">|</span><svg class="meta_icon post-meta-icon" style="width:18px;height:18px;position:relative;top:5px"><use xlink:href="#icon-gengxin1"></use></svg><span class="post-meta-label">更新于</span><time class="post-meta-date-updated" datetime="2024-04-26T09:52:18.854Z" title="更新于 2024-04-26 17:52:18">2024-04-26</time></span><span class="post-meta-categories"><span class="post-meta-separator">|</span><svg class="meta_icon post-meta-icon" style="width:18px;height:18px;position:relative;top:5px"><use xlink:href="#icon-biaoqian"></use></svg><a class="post-meta-categories" href="/categories/%E4%B8%AD%E9%97%B4%E4%BB%B6/">中间件</a><i class="fas fa-angle-right post-meta-separator"></i><svg class="meta_icon post-meta-icon" style="width:18px;height:18px;position:relative;top:5px"><use xlink:href="#icon-biaoqian"></use></svg><a class="post-meta-categories" href="/categories/%E4%B8%AD%E9%97%B4%E4%BB%B6/MySQL/">MySQL</a></span></div><div class="meta-secondline"><span class="post-meta-separator">|</span><span class="post-meta-wordcount"><svg class="meta_icon post-meta-icon" style="width:25px;height:25px;position:relative;top:8px"><use xlink:href="#icon-charuword"></use></svg><span class="post-meta-label">字数总计:</span><span class="word-count">4184</span><span class="post-meta-separator">|</span><svg class="meta_icon post-meta-icon" style="width:20px;height:20px;position:relative;top:5px"><use xlink:href="#icon-shizhong"></use></svg><span class="post-meta-label">阅读时长:</span><span>15分钟</span></span><span class="post-meta-separator">|</span><span class="post-meta-pv-cv" id="" data-flag-title="第六-MySQL多表查询"><svg class="meta_icon post-meta-icon" style="width:25px;height:25px;position:relative;top:5px"><use xlink:href="#icon-eye"></use></svg><span class="post-meta-label">阅读量:</span><span id="busuanzi_value_page_pv"><i class="fa-solid fa-spinner fa-spin"></i></span></span></div></div></div><section class="main-hero-waves-area waves-area"><svg class="waves-svg" xmlns="http://www.w3.org/2000/svg" xlink="http://www.w3.org/1999/xlink" viewBox="0 24 150 28" preserveAspectRatio="none" shape-rendering="auto"><defs><path id="gentle-wave" d="M -160 44 c 30 0 58 -18 88 -18 s 58 18 88 18 s 58 -18 88 -18 s 58 18 88 18 v 44 h -352 Z"></path></defs><g class="parallax"><use href="#gentle-wave" x="48" y="0"></use><use href="#gentle-wave" x="48" y="3"></use><use href="#gentle-wave" x="48" y="5"></use><use href="#gentle-wave" x="48" y="7"></use></g></svg></section></header><main class="layout" id="content-inner"><div id="post"><article class="post-content" id="article-container"><h1 id="第06章-多表查询"><a href="#第06章-多表查询" class="headerlink" title="第06章_多表查询"></a>第06章_多表查询</h1><p>多表查询，也称为关联查询，指两个或更多个表一起完成查询操作。</p>
<p>前提条件：这些一起查询的表之间是有关系的（一对一、一对多），它们之间一定是有关联字段，这个关联字段可能建立了外键，也可能没有建立外键。比如：员工表和部门表，这两个表依靠“部门编号”进行关联。</p>
<h2 id="1-一个案例引发的多表连接"><a href="#1-一个案例引发的多表连接" class="headerlink" title="1. 一个案例引发的多表连接"></a>1. 一个案例引发的多表连接</h2><h3 id="1-1-案例说明"><a href="#1-1-案例说明" class="headerlink" title="1.1 案例说明"></a>1.1 案例说明</h3><p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554974984600.webp" alt="1554974984600"></p>
<p>从多个表中获取数据：</p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554975020388.webp" alt="1554975020388"></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">#案例：查询员工的姓名及其部门名称</span><br><span class="line">SELECT last_name, department_name</span><br><span class="line">FROM employees, departments;</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554975097631.webp" alt="1554975097631"></p>
<p>查询结果：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line">+-----------+----------------------+</span><br><span class="line">| last_name | department_name      |</span><br><span class="line">+-----------+----------------------+</span><br><span class="line">| King      | Administration       |</span><br><span class="line">| King      | Marketing            |</span><br><span class="line">| King      | Purchasing           |</span><br><span class="line">| King      | Human Resources      |</span><br><span class="line">| King      | Shipping             |</span><br><span class="line">| King      | IT                   |</span><br><span class="line">| King      | Public Relations     |</span><br><span class="line">| King      | Sales                |</span><br><span class="line">| King      | Executive            |</span><br><span class="line">| King      | Finance              |</span><br><span class="line">| King      | Accounting           |</span><br><span class="line">| King      | Treasury             |</span><br><span class="line">...</span><br><span class="line">| Gietz     | IT Support           |</span><br><span class="line">| Gietz     | NOC                  |</span><br><span class="line">| Gietz     | IT Helpdesk          |</span><br><span class="line">| Gietz     | Government Sales     |</span><br><span class="line">| Gietz     | Retail Sales         |</span><br><span class="line">| Gietz     | Recruiting           |</span><br><span class="line">| Gietz     | Payroll              |</span><br><span class="line">+-----------+----------------------+</span><br><span class="line">2889 rows in set (0.01 sec)</span><br></pre></td></tr></table></figure>
<p><strong>分析错误情况：</strong></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">SELECT COUNT(employee_id) FROM employees;</span><br><span class="line">#输出107行</span><br><span class="line"></span><br><span class="line">SELECT COUNT(department_id)FROM departments;</span><br><span class="line">#输出27行</span><br><span class="line"></span><br><span class="line">SELECT 107*27 FROM dual;</span><br></pre></td></tr></table></figure>
<p>我们把上述多表查询中出现的问题称为：笛卡尔积的错误。</p>
<h3 id="1-2-笛卡尔积（或交叉连接）的理解"><a href="#1-2-笛卡尔积（或交叉连接）的理解" class="headerlink" title="1.2 笛卡尔积（或交叉连接）的理解"></a>1.2 笛卡尔积（或交叉连接）的理解</h3><p>笛卡尔乘积是一个数学运算。假设我有两个集合 X 和 Y，那么 X 和 Y 的笛卡尔积就是 X 和 Y 的所有可能组合，也就是第一个对象来自于 X，第二个对象来自于 Y 的所有可能。组合的个数即为两个集合中元素个数的乘积数。</p>
<p><img src= "" data-lazy-src="https://images0.cnblogs.com/i/595518/201403/302046364841977.webp" alt="img" style="zoom:80%;" /></p>
<p>SQL92中，笛卡尔积也称为<code>交叉连接</code>，英文是 <code>CROSS JOIN</code>。在 SQL99 中也是使用 CROSS JOIN表示交叉连接。它的作用就是可以把任意表进行连接，即使这两张表不相关。在MySQL中如下情况会出现笛卡尔积：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">#查询员工姓名和所在部门名称</span><br><span class="line">SELECT last_name,department_name FROM employees,departments;</span><br><span class="line">SELECT last_name,department_name FROM employees CROSS JOIN departments;</span><br><span class="line">SELECT last_name,department_name FROM employees INNER JOIN departments;</span><br><span class="line">SELECT last_name,department_name FROM employees JOIN departments;</span><br></pre></td></tr></table></figure>
<h3 id="1-3-案例分析与问题解决"><a href="#1-3-案例分析与问题解决" class="headerlink" title="1.3 案例分析与问题解决"></a>1.3 案例分析与问题解决</h3><ul>
<li><p><strong>笛卡尔积的错误会在下面条件下产生</strong>：</p>
<ul>
<li>省略多个表的连接条件（或关联条件）</li>
<li>连接条件（或关联条件）无效</li>
<li>所有表中的所有行互相连接</li>
</ul>
</li>
<li><p>为了避免笛卡尔积， 可以<strong>在 WHERE 加入有效的连接条件。</strong></p>
</li>
<li><p>加入连接条件后，查询语法：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">SELECT	table1.column, table2.column</span><br><span class="line">FROM	table1, table2</span><br><span class="line">WHERE	table1.column1 = table2.column2;  #连接条件</span><br></pre></td></tr></table></figure>
<ul>
<li><strong>在 WHERE子句中写入连接条件。</strong></li>
</ul>
</li>
<li><p>正确写法：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">#案例：查询员工的姓名及其部门名称</span><br><span class="line">SELECT last_name, department_name</span><br><span class="line">FROM employees, departments</span><br><span class="line">WHERE employees.department_id = departments.department_id;</span><br></pre></td></tr></table></figure>
</li>
<li><p><strong>在表中有相同列时，在列名之前加上表名前缀。</strong></p>
</li>
</ul>
<h2 id="2-多表查询分类讲解"><a href="#2-多表查询分类讲解" class="headerlink" title="2. 多表查询分类讲解"></a>2. 多表查询分类讲解</h2><h3 id="分类1：等值连接-vs-非等值连接"><a href="#分类1：等值连接-vs-非等值连接" class="headerlink" title="分类1：等值连接 vs 非等值连接"></a>分类1：等值连接 vs 非等值连接</h3><h4 id="等值连接"><a href="#等值连接" class="headerlink" title="等值连接"></a>等值连接</h4><p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554975496900.webp" alt="1554975496900"></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">SELECT employees.employee_id, employees.last_name, </span><br><span class="line">       employees.department_id, departments.department_id,</span><br><span class="line">       departments.location_id</span><br><span class="line">FROM   employees, departments</span><br><span class="line">WHERE  employees.department_id = departments.department_id;</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554975522600.webp" alt="1554975522600"></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554975526339.webp" alt="1554975526339"></p>
<p><strong>拓展1：多个连接条件与 AND 操作符</strong> </p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554975606231.webp" alt="1554975606231"></p>
<p><strong>拓展2：区分重复的列名</strong></p>
<ul>
<li><strong>多个表中有相同列时，必须在列名之前加上表名前缀。</strong></li>
<li>在不同表中具有相同列名的列可以用<code>表名</code>加以区分。</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">SELECT employees.last_name, departments.department_name,employees.department_id</span><br><span class="line">FROM employees, departments</span><br><span class="line">WHERE employees.department_id = departments.department_id;</span><br></pre></td></tr></table></figure>
<p><strong>拓展3：表的别名</strong></p>
<ul>
<li><p>使用别名可以简化查询。</p>
</li>
<li><p>列名前使用表名前缀可以提高查询效率。</p>
</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">SELECT e.employee_id, e.last_name, e.department_id,</span><br><span class="line">       d.department_id, d.location_id</span><br><span class="line">FROM   employees e , departments d</span><br><span class="line">WHERE  e.department_id = d.department_id;</span><br></pre></td></tr></table></figure>
<blockquote>
<p>需要注意的是，如果我们使用了表的别名，在查询字段中、过滤条件中就只能使用别名进行代替，不能使用原有的表名，否则就会报错。</p>
<p><code>阿里开发规范</code>：</p>
<p>【<code>强制</code>】对于数据库中表记录的查询和变更，只要涉及多个表，都需要在列名前加表的别名（或 表名）进行限定。 </p>
<p><code>说明</code>：对多表进行查询记录、更新记录、删除记录时，如果对操作列没有限定表的别名（或表名），并且操作列在多个表中存在时，就会抛异常。 </p>
<p><code>正例</code>：select t1.name from table_first as t1 , table_second as t2 where t1.id=t2.id; </p>
<p><code>反例</code>：在某业务中，由于多表关联查询语句没有加表的别名（或表名）的限制，正常运行两年后，最近在 某个表中增加一个同名字段，在预发布环境做数据库变更后，线上查询语句出现出 1052 异常：Column  ‘name’ in field list is ambiguous。</p>
</blockquote>
<p><strong>拓展4：连接多个表</strong></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554978354431.webp" alt="1554978354431"></p>
<p><strong>总结：连接 n个表,至少需要n-1个连接条件。</strong>比如，连接三个表，至少需要两个连接条件。</p>
<p>练习：查询出公司员工的 last_name,department_name, city</p>
<h4 id="非等值连接"><a href="#非等值连接" class="headerlink" title="非等值连接"></a>非等值连接</h4><p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554978442447.webp" alt="1554978442447"></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">SELECT e.last_name, e.salary, j.grade_level</span><br><span class="line">FROM   employees e, job_grades j</span><br><span class="line">WHERE  e.salary BETWEEN j.lowest_sal AND j.highest_sal;</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554978477013.webp" alt="1554978477013"></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554978482652.webp" alt="1554978482652"></p>
<h3 id="分类2：自连接-vs-非自连接"><a href="#分类2：自连接-vs-非自连接" class="headerlink" title="分类2：自连接 vs 非自连接"></a>分类2：自连接 vs 非自连接</h3><p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554978514321.webp" alt="1554978514321"></p>
<ul>
<li>当table1和table2本质上是同一张表，只是用取别名的方式虚拟成两张表以代表不同的意义。然后两个表再进行内连接，外连接等查询。</li>
</ul>
<p><strong>题目：查询employees表，返回“Xxx  works for Xxx”</strong></p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">SELECT CONCAT(worker.last_name ,&#x27; works for &#x27; </span><br><span class="line">       , manager.last_name)</span><br><span class="line">FROM   employees worker, employees manager</span><br><span class="line">WHERE  worker.manager_id = manager.employee_id ;</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554978684947.webp" alt="1554978684947"></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554978690764.webp" alt="1554978690764"></p>
<p>练习：查询出last_name为 ‘Chen’ 的员工的 manager 的信息。</p>
<h3 id="分类3：内连接-vs-外连接"><a href="#分类3：内连接-vs-外连接" class="headerlink" title="分类3：内连接 vs 外连接"></a>分类3：内连接 vs 外连接</h3><p>除了查询满足条件的记录以外，外连接还可以查询某一方不满足条件的记录。</p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554978955659.webp" alt="1554978955659"></p>
<ul>
<li><p>内连接: 合并具有同一列的两个以上的表的行, <strong>结果集中不包含一个表与另一个表不匹配的行</strong></p>
</li>
<li><p>外连接: 两个表在连接过程中除了返回满足连接条件的行以外<strong>还返回左（或右）表中不满足条件的行</strong> <strong>，这种连接称为左（或右） 外连接</strong>。没有匹配的行时, 结果表中相应的列为空(NULL)。</p>
</li>
<li><p>如果是左外连接，则连接条件中左边的表也称为<code>主表</code>，右边的表称为<code>从表</code>。</p>
<p>如果是右外连接，则连接条件中右边的表也称为<code>主表</code>，左边的表称为<code>从表</code>。</p>
</li>
</ul>
<h4 id="SQL92：使用-创建连接"><a href="#SQL92：使用-创建连接" class="headerlink" title="SQL92：使用(+)创建连接"></a>SQL92：使用(+)创建连接</h4><ul>
<li><p>在 SQL92 中采用（+）代表从表所在的位置。即左或右外连接中，(+) 表示哪个是从表。</p>
</li>
<li><p>Oracle 对 SQL92 支持较好，而 MySQL 则不支持 SQL92 的外连接。</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">#左外连接</span><br><span class="line">SELECT last_name,department_name</span><br><span class="line">FROM employees ,departments</span><br><span class="line">WHERE employees.department_id = departments.department_id(+);</span><br><span class="line"></span><br><span class="line">#右外连接</span><br><span class="line">SELECT last_name,department_name</span><br><span class="line">FROM employees ,departments</span><br><span class="line">WHERE employees.department_id(+) = departments.department_id;</span><br></pre></td></tr></table></figure>
</li>
<li><p>而且在 SQL92 中，只有左外连接和右外连接，没有满（或全）外连接。</p>
</li>
</ul>
<h2 id="3-SQL99语法实现多表查询"><a href="#3-SQL99语法实现多表查询" class="headerlink" title="3. SQL99语法实现多表查询"></a>3. SQL99语法实现多表查询</h2><h3 id="3-1-基本语法"><a href="#3-1-基本语法" class="headerlink" title="3.1 基本语法"></a>3.1 基本语法</h3><ul>
<li><p>使用JOIN…ON子句创建连接的语法结构：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">SELECT table1.column, table2.column,table3.column</span><br><span class="line">FROM table1</span><br><span class="line">    JOIN table2 ON table1 和 table2 的连接条件</span><br><span class="line">        JOIN table3 ON table2 和 table3 的连接条件</span><br></pre></td></tr></table></figure>
<p>它的嵌套逻辑类似我们使用的 FOR 循环：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">for t1 in table1:</span><br><span class="line">    for t2 in table2:</span><br><span class="line">       if condition1:</span><br><span class="line">           for t3 in table3:</span><br><span class="line">              if condition2:</span><br><span class="line">                  output t1 + t2 + t3</span><br></pre></td></tr></table></figure>
<p>SQL99 采用的这种嵌套结构非常清爽、层次性更强、可读性更强，即使再多的表进行连接也都清晰可见。如果你采用 SQL92，可读性就会大打折扣。</p>
</li>
<li><p>语法说明：</p>
<ul>
<li><strong>可以使用</strong> <strong>ON</strong> <strong>子句指定额外的连接条件</strong>。</li>
<li>这个连接条件是与其它条件分开的。</li>
<li><strong>ON</strong> <strong>子句使语句具有更高的易读性</strong>。</li>
<li>关键字 JOIN、INNER JOIN、CROSS JOIN 的含义是一样的，都表示内连接</li>
</ul>
</li>
</ul>
<h3 id="3-2-内连接-INNER-JOIN-的实现"><a href="#3-2-内连接-INNER-JOIN-的实现" class="headerlink" title="3.2 内连接(INNER JOIN)的实现"></a>3.2 内连接(INNER JOIN)的实现</h3><ul>
<li>语法：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">SELECT 字段列表</span><br><span class="line">FROM A表 INNER JOIN B表</span><br><span class="line">ON 关联条件</span><br><span class="line">WHERE 等其他子句;</span><br></pre></td></tr></table></figure>
<p>题目1：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">SELECT e.employee_id, e.last_name, e.department_id, </span><br><span class="line">       d.department_id, d.location_id</span><br><span class="line">FROM   employees e JOIN departments d</span><br><span class="line">ON     (e.department_id = d.department_id);</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979073996.webp" alt="1554979073996"></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979079395.webp" alt="1554979079395"></p>
<p>题目2：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">SELECT employee_id, city, department_name</span><br><span class="line">FROM   employees e </span><br><span class="line">JOIN   departments d</span><br><span class="line">ON     d.department_id = e.department_id </span><br><span class="line">JOIN   locations l</span><br><span class="line">ON     d.location_id = l.location_id;</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979110008.webp" alt="1554979110008"></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979115642.webp" alt="1554979115642"></p>
<h3 id="3-3-外连接-OUTER-JOIN-的实现"><a href="#3-3-外连接-OUTER-JOIN-的实现" class="headerlink" title="3.3 外连接(OUTER JOIN)的实现"></a>3.3 外连接(OUTER JOIN)的实现</h3><h4 id="3-3-1-左外连接-LEFT-OUTER-JOIN"><a href="#3-3-1-左外连接-LEFT-OUTER-JOIN" class="headerlink" title="3.3.1 左外连接(LEFT OUTER JOIN)"></a>3.3.1 左外连接(LEFT OUTER JOIN)</h4><ul>
<li>语法：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">#实现查询结果是A</span><br><span class="line">SELECT 字段列表</span><br><span class="line">FROM A表 LEFT JOIN B表</span><br><span class="line">ON 关联条件</span><br><span class="line">WHERE 等其他子句;</span><br></pre></td></tr></table></figure>
<ul>
<li>举例：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">SELECT e.last_name, e.department_id, d.department_name</span><br><span class="line">FROM   employees e</span><br><span class="line">LEFT OUTER JOIN departments d</span><br><span class="line">ON   (e.department_id = d.department_id) ;</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979200961.webp" alt="1554979200961"></p>
<h4 id="3-3-2-右外连接-RIGHT-OUTER-JOIN"><a href="#3-3-2-右外连接-RIGHT-OUTER-JOIN" class="headerlink" title="3.3.2 右外连接(RIGHT OUTER JOIN)"></a>3.3.2 右外连接(RIGHT OUTER JOIN)</h4><ul>
<li>语法：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">#实现查询结果是B</span><br><span class="line">SELECT 字段列表</span><br><span class="line">FROM A表 RIGHT JOIN B表</span><br><span class="line">ON 关联条件</span><br><span class="line">WHERE 等其他子句;</span><br></pre></td></tr></table></figure>
<ul>
<li>举例：</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">SELECT e.last_name, e.department_id, d.department_name</span><br><span class="line">FROM   employees e</span><br><span class="line">RIGHT OUTER JOIN departments d</span><br><span class="line">ON    (e.department_id = d.department_id) ;</span><br></pre></td></tr></table></figure>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979243194.webp" alt="1554979243194"></p>
<blockquote>
<p>需要注意的是，LEFT JOIN 和 RIGHT JOIN 只存在于 SQL99 及以后的标准中，在 SQL92 中不存在，只能用 (+) 表示。</p>
</blockquote>
<h4 id="3-3-3-满外连接-FULL-OUTER-JOIN"><a href="#3-3-3-满外连接-FULL-OUTER-JOIN" class="headerlink" title="3.3.3 满外连接(FULL OUTER JOIN)"></a>3.3.3 满外连接(FULL OUTER JOIN)</h4><ul>
<li>满外连接的结果 = 左右表匹配的数据 + 左表没有匹配到的数据 + 右表没有匹配到的数据。</li>
<li>SQL99是支持满外连接的。使用FULL JOIN 或 FULL OUTER JOIN来实现。</li>
<li>需要注意的是，MySQL不支持FULL JOIN，但是可以用 LEFT JOIN <strong>UNION</strong> RIGHT join代替。</li>
</ul>
<h2 id="4-UNION的使用"><a href="#4-UNION的使用" class="headerlink" title="4. UNION的使用"></a>4. UNION的使用</h2><p><strong>合并查询结果</strong><br>利用UNION关键字，可以给出多条SELECT语句，并将它们的结果组合成单个结果集。合并时，两个表对应的列数和数据类型必须相同，并且相互对应。各个SELECT语句之间使用UNION或UNION ALL关键字分隔。</p>
<p>语法格式：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">SELECT column,... FROM table1</span><br><span class="line">UNION [ALL]</span><br><span class="line">SELECT column,... FROM table2</span><br></pre></td></tr></table></figure>
<p><strong>UNION操作符</strong></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979317187.webp" alt="1554979317187" style="zoom: 67%;" /></p>
<p>UNION 操作符返回两个查询的结果集的并集，去除重复记录。</p>
<p><strong>UNION ALL操作符</strong></p>
<p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979343634.webp" alt="1554979343634" style="zoom: 67%;" /></p>
<p>UNION ALL操作符返回两个查询的结果集的并集。对于两个结果集的重复部分，不去重。</p>
<blockquote>
<p>注意：执行UNION ALL语句时所需要的资源比UNION语句少。如果明确知道合并数据后的结果数据不存在重复数据，或者不需要去除重复的数据，则尽量使用UNION ALL语句，以提高数据查询的效率。</p>
</blockquote>
<p>举例：查询部门编号&gt;90或邮箱包含a的员工信息</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">#方式1</span><br><span class="line">SELECT * FROM employees WHERE email LIKE &#x27;%a%&#x27; OR department_id&gt;90;</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">#方式2</span><br><span class="line">SELECT * FROM employees  WHERE email LIKE &#x27;%a%&#x27;</span><br><span class="line">UNION</span><br><span class="line">SELECT * FROM employees  WHERE department_id&gt;90;</span><br></pre></td></tr></table></figure>
<p>举例：查询中国用户中男性的信息以及美国用户中年男性的用户信息</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">SELECT id,cname FROM t_chinamale WHERE csex=&#x27;男&#x27;</span><br><span class="line">UNION ALL</span><br><span class="line">SELECT id,tname FROM t_usmale WHERE tGender=&#x27;male&#x27;;</span><br></pre></td></tr></table></figure>
<h2 id="5-7种SQL-JOINS的实现"><a href="#5-7种SQL-JOINS的实现" class="headerlink" title="5. 7种SQL JOINS的实现"></a>5. 7种SQL JOINS的实现</h2><p><img src= "" data-lazy-src="https://www.naste.top:9000/webp/1554979255233.webp" alt="1554979255233" style="zoom:80%;" /></p>
<h3 id="5-7-1-代码实现"><a href="#5-7-1-代码实现" class="headerlink" title="5.7.1 代码实现"></a>5.7.1 代码实现</h3><figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">#中图：内连接 A∩B</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`;</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">#左上图：左外连接</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e LEFT JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`;</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">#右上图：右外连接</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e RIGHT JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`;</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">#左中图：A - A∩B</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e LEFT JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`</span><br><span class="line">WHERE d.`department_id` IS NULL</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">#右中图：B-A∩B</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e RIGHT JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`</span><br><span class="line">WHERE e.`department_id` IS NULL</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">#左下图：满外连接</span><br><span class="line"># 左中图 + 右上图  A∪B</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e LEFT JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`</span><br><span class="line">WHERE d.`department_id` IS NULL</span><br><span class="line">UNION ALL  #没有去重操作，效率高</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e RIGHT JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`;</span><br></pre></td></tr></table></figure>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line">#右下图</span><br><span class="line">#左中图 + 右中图  A ∪B- A∩B 或者 (A -  A∩B) ∪ （B - A∩B）</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e LEFT JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`</span><br><span class="line">WHERE d.`department_id` IS NULL</span><br><span class="line">UNION ALL</span><br><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e RIGHT JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`</span><br><span class="line">WHERE e.`department_id` IS NULL</span><br></pre></td></tr></table></figure>
<h3 id="5-7-2-语法格式小结"><a href="#5-7-2-语法格式小结" class="headerlink" title="5.7.2 语法格式小结"></a>5.7.2 语法格式小结</h3><ul>
<li>左中图</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">#实现A -  A∩B</span><br><span class="line">select 字段列表</span><br><span class="line">from A表 left join B表</span><br><span class="line">on 关联条件</span><br><span class="line">where 从表关联字段 is null and 等其他子句;</span><br></pre></td></tr></table></figure>
<ul>
<li>右中图</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">#实现B -  A∩B</span><br><span class="line">select 字段列表</span><br><span class="line">from A表 right join B表</span><br><span class="line">on 关联条件</span><br><span class="line">where 从表关联字段 is null and 等其他子句;</span><br></pre></td></tr></table></figure>
<ul>
<li>左下图</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">#实现查询结果是A∪B</span><br><span class="line">#用左外的A，union 右外的B</span><br><span class="line">select 字段列表</span><br><span class="line">from A表 left join B表</span><br><span class="line">on 关联条件</span><br><span class="line">where 等其他子句</span><br><span class="line"></span><br><span class="line">union </span><br><span class="line"></span><br><span class="line">select 字段列表</span><br><span class="line">from A表 right join B表</span><br><span class="line">on 关联条件</span><br><span class="line">where 等其他子句;</span><br></pre></td></tr></table></figure>
<ul>
<li>右下图</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line">#实现A∪B -  A∩B  或   (A -  A∩B) ∪ （B - A∩B）</span><br><span class="line">#使用左外的 (A -  A∩B)  union 右外的（B - A∩B）</span><br><span class="line">select 字段列表</span><br><span class="line">from A表 left join B表</span><br><span class="line">on 关联条件</span><br><span class="line">where 从表关联字段 is null and 等其他子句</span><br><span class="line"></span><br><span class="line">union</span><br><span class="line"></span><br><span class="line">select 字段列表</span><br><span class="line">from A表 right join B表</span><br><span class="line">on 关联条件</span><br><span class="line">where 从表关联字段 is null and 等其他子句</span><br></pre></td></tr></table></figure>
<h2 id="6-SQL99语法新特性"><a href="#6-SQL99语法新特性" class="headerlink" title="6. SQL99语法新特性"></a>6. SQL99语法新特性</h2><h3 id="6-1-自然连接"><a href="#6-1-自然连接" class="headerlink" title="6.1 自然连接"></a>6.1 自然连接</h3><p>SQL99 在 SQL92 的基础上提供了一些特殊语法，比如 <code>NATURAL JOIN</code> 用来表示自然连接。我们可以把自然连接理解为 SQL92 中的等值连接。它会帮你自动查询两张连接表中<code>所有相同的字段</code>，然后进行<code>等值连接</code>。</p>
<p>在SQL92标准中：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e JOIN departments d</span><br><span class="line">ON e.`department_id` = d.`department_id`</span><br><span class="line">AND e.`manager_id` = d.`manager_id`;</span><br></pre></td></tr></table></figure>
<p>在 SQL99 中你可以写成：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e NATURAL JOIN departments d;</span><br></pre></td></tr></table></figure>
<h3 id="6-2-USING连接"><a href="#6-2-USING连接" class="headerlink" title="6.2 USING连接"></a>6.2 USING连接</h3><p>当我们进行连接的时候，SQL99还支持使用 USING 指定数据表里的<code>同名字段</code>进行等值连接。但是只能配合JOIN一起使用。比如：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e JOIN departments d</span><br><span class="line">USING (department_id);</span><br></pre></td></tr></table></figure>
<p>你能看出与自然连接 NATURAL JOIN 不同的是，USING 指定了具体的相同的字段名称，你需要在 USING 的括号 () 中填入要指定的同名字段。同时使用 <code>JOIN...USING</code> 可以简化 JOIN ON 的等值连接。它与下面的 SQL 查询结果是相同的：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">SELECT employee_id,last_name,department_name</span><br><span class="line">FROM employees e ,departments d</span><br><span class="line">WHERE e.department_id = d.department_id;</span><br></pre></td></tr></table></figure>
<h2 id="7-章节小结"><a href="#7-章节小结" class="headerlink" title="7. 章节小结"></a>7. 章节小结</h2><p>表连接的约束条件可以有三种方式：WHERE, ON, USING</p>
<ul>
<li><p>WHERE：适用于所有关联查询</p>
</li>
<li><p><code>ON</code>：只能和JOIN一起使用，只能写关联条件。虽然关联条件可以并到WHERE中和其他条件一起写，但分开写可读性更好。</p>
</li>
<li><p>USING：只能和JOIN一起使用，而且要求<strong>两个</strong>关联字段在关联表中名称一致，而且只能表示关联字段值相等</p>
</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br></pre></td><td class="code"><pre><span class="line">#关联条件</span><br><span class="line">#把关联条件写在where后面</span><br><span class="line">SELECT last_name,department_name </span><br><span class="line">FROM employees,departments </span><br><span class="line">WHERE employees.department_id = departments.department_id;</span><br><span class="line"></span><br><span class="line">#把关联条件写在on后面，只能和JOIN一起使用</span><br><span class="line">SELECT last_name,department_name </span><br><span class="line">FROM employees INNER JOIN departments </span><br><span class="line">ON employees.department_id = departments.department_id;</span><br><span class="line"></span><br><span class="line">SELECT last_name,department_name </span><br><span class="line">FROM employees CROSS JOIN departments </span><br><span class="line">ON employees.department_id = departments.department_id;</span><br><span class="line"></span><br><span class="line">SELECT last_name,department_name  </span><br><span class="line">FROM employees JOIN departments </span><br><span class="line">ON employees.department_id = departments.department_id;</span><br><span class="line"></span><br><span class="line">#把关联字段写在using()中，只能和JOIN一起使用</span><br><span class="line">#而且两个表中的关联字段必须名称相同，而且只能表示=</span><br><span class="line">#查询员工姓名与基本工资</span><br><span class="line">SELECT last_name,job_title</span><br><span class="line">FROM employees INNER JOIN jobs USING(job_id);</span><br><span class="line"></span><br><span class="line">#n张表关联，需要n-1个关联条件</span><br><span class="line">#查询员工姓名，基本工资，部门名称</span><br><span class="line">SELECT last_name,job_title,department_name FROM employees,departments,jobs </span><br><span class="line">WHERE employees.department_id = departments.department_id </span><br><span class="line">AND employees.job_id = jobs.job_id;</span><br><span class="line"></span><br><span class="line">SELECT last_name,job_title,department_name </span><br><span class="line">FROM employees INNER JOIN departments INNER JOIN jobs </span><br><span class="line">ON employees.department_id = departments.department_id </span><br><span class="line">AND employees.job_id = jobs.job_id;</span><br></pre></td></tr></table></figure>
<p><strong>注意：</strong></p>
<p>我们要<code>控制连接表的数量</code>。多表连接就相当于嵌套 for 循环一样，非常消耗资源，会让 SQL 查询性能下降得很严重，因此不要连接不必要的表。在许多 DBMS 中，也都会有最大连接表的限制。</p>
<blockquote>
<p>【强制】超过三个表禁止 join。需要 join 的字段，数据类型保持绝对一致；多表关联查询时， 保证被关联的字段需要有索引。 </p>
<p>说明：即使双表 join 也要注意表索引、SQL 性能。</p>
<p>来源：阿里巴巴《Java开发手册》</p>
</blockquote>
<h2 id="附录：常用的-SQL-标准有哪些"><a href="#附录：常用的-SQL-标准有哪些" class="headerlink" title="附录：常用的 SQL 标准有哪些"></a>附录：常用的 SQL 标准有哪些</h2><p>在正式开始讲连接表的种类时，我们首先需要知道 SQL 存在不同版本的标准规范，因为不同规范下的表连接操作是有区别的。</p>
<p>SQL 有两个主要的标准，分别是 <code>SQL92</code> 和 <code>SQL99</code>。92 和 99 代表了标准提出的时间，SQL92 就是 92 年提出的标准规范。当然除了 SQL92 和 SQL99 以外，还存在 SQL-86、SQL-89、SQL:2003、SQL:2008、SQL:2011 和 SQL:2016 等其他的标准。</p>
<p>这么多标准，到底该学习哪个呢？<strong>实际上最重要的 SQL 标准就是 SQL92 和 SQL99</strong>。一般来说 SQL92 的形式更简单，但是写的 SQL 语句会比较长，可读性较差。而 SQL99 相比于 SQL92 来说，语法更加复杂，但可读性更强。我们从这两个标准发布的页数也能看出，SQL92 的标准有 500 页，而 SQL99 标准超过了 1000 页。实际上从 SQL99 之后，很少有人能掌握所有内容，因为确实太多了。就好比我们使用 Windows、Linux 和 Office 的时候，很少有人能掌握全部内容一样。我们只需要掌握一些核心的功能，满足日常工作的需求即可。</p>
<p><strong>SQL92 和 SQL99 是经典的 SQL 标准，也分别叫做 SQL-2 和 SQL-3 标准。</strong>也正是在这两个标准发布之后，SQL 影响力越来越大，甚至超越了数据库领域。现如今 SQL 已经不仅仅是数据库领域的主流语言，还是信息领域中信息处理的主流语言。在图形检索、图像检索以及语音检索中都能看到 SQL 语言的使用。</p>
</article><div class="post-copyright"><div class="post-copyright__title"><span class="post-copyright-info"><h>第六-MySQL多表查询</h></span></div><div class="post-copyright__type"><span class="post-copyright-info"><a href="https://www.naste.top:1024/posts/1372804151.html">https://www.naste.top:1024/posts/1372804151.html</a></span></div><div class="post-copyright-m"><div class="post-copyright-m-info"><div class="post-copyright-a"><h>作者</h><div class="post-copyright-cc-info"><h>XiaoFei🥝</h></div></div><div class="post-copyright-c"><h>发布于</h><div class="post-copyright-cc-info"><h>2022-09-19</h></div></div><div class="post-copyright-u"><h>更新于</h><div class="post-copyright-cc-info"><h>2024-04-26</h></div></div><div class="post-copyright-c"><h>许可协议</h><div class="post-copyright-cc-info"><a class="icon" rel="noopener" target="_blank" title="Creative Commons" href="https://creativecommons.org/"><i class="fab fa-creative-commons"></i></a><a rel="noopener" target="_blank" title="CC BY-NC-SA 4.0" 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2024-04-26</div><div class="title">MySQL基本使用</div></div></a></div><div><a href="/posts/2844609012.html" title="第零-MySQL写在前面"><img class="cover" src= "" data-lazy-src="https://www.naste.top:9000/webp/1675912181.webp" alt="cover"><div class="content is-center"><div class="date"><i class="fas fa-history fa-fw"></i> 2024-04-26</div><div class="title">第零-MySQL写在前面</div></div></a></div></div></div><hr/><div id="post-comment"><div class="comment-head"><div class="comment-headline"><i class="fas fa-comments fa-fw"></i><span> 评论</span></div></div><div class="comment-wrap"><div><div id="twikoo-wrap"></div></div></div></div></div><div class="aside-content" id="aside-content"><div class="sticky_layout"><div class="card-widget" id="card-toc"><div class="item-headline"><svg class="meta_icon" style="width:22px;height:22px;position:relative;top:5px"><use xlink:href="#icon-mulu1"></use></svg><span style="font-weight:bold">目录</span><span class="toc-percentage"></span></div><div class="toc-content"><ol class="toc"><li class="toc-item toc-level-1"><a class="toc-link" href="#%E7%AC%AC06%E7%AB%A0-%E5%A4%9A%E8%A1%A8%E6%9F%A5%E8%AF%A2"><span class="toc-text">第06章_多表查询</span></a><ol class="toc-child"><li class="toc-item toc-level-2"><a class="toc-link" href="#1-%E4%B8%80%E4%B8%AA%E6%A1%88%E4%BE%8B%E5%BC%95%E5%8F%91%E7%9A%84%E5%A4%9A%E8%A1%A8%E8%BF%9E%E6%8E%A5"><span class="toc-text">1. 一个案例引发的多表连接</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#1-1-%E6%A1%88%E4%BE%8B%E8%AF%B4%E6%98%8E"><span class="toc-text">1.1 案例说明</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#1-2-%E7%AC%9B%E5%8D%A1%E5%B0%94%E7%A7%AF%EF%BC%88%E6%88%96%E4%BA%A4%E5%8F%89%E8%BF%9E%E6%8E%A5%EF%BC%89%E7%9A%84%E7%90%86%E8%A7%A3"><span class="toc-text">1.2 笛卡尔积（或交叉连接）的理解</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#1-3-%E6%A1%88%E4%BE%8B%E5%88%86%E6%9E%90%E4%B8%8E%E9%97%AE%E9%A2%98%E8%A7%A3%E5%86%B3"><span class="toc-text">1.3 案例分析与问题解决</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#2-%E5%A4%9A%E8%A1%A8%E6%9F%A5%E8%AF%A2%E5%88%86%E7%B1%BB%E8%AE%B2%E8%A7%A3"><span class="toc-text">2. 多表查询分类讲解</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%88%86%E7%B1%BB1%EF%BC%9A%E7%AD%89%E5%80%BC%E8%BF%9E%E6%8E%A5-vs-%E9%9D%9E%E7%AD%89%E5%80%BC%E8%BF%9E%E6%8E%A5"><span class="toc-text">分类1：等值连接 vs 非等值连接</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#%E7%AD%89%E5%80%BC%E8%BF%9E%E6%8E%A5"><span class="toc-text">等值连接</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E9%9D%9E%E7%AD%89%E5%80%BC%E8%BF%9E%E6%8E%A5"><span class="toc-text">非等值连接</span></a></li></ol></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%88%86%E7%B1%BB2%EF%BC%9A%E8%87%AA%E8%BF%9E%E6%8E%A5-vs-%E9%9D%9E%E8%87%AA%E8%BF%9E%E6%8E%A5"><span class="toc-text">分类2：自连接 vs 非自连接</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%88%86%E7%B1%BB3%EF%BC%9A%E5%86%85%E8%BF%9E%E6%8E%A5-vs-%E5%A4%96%E8%BF%9E%E6%8E%A5"><span class="toc-text">分类3：内连接 vs 外连接</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#SQL92%EF%BC%9A%E4%BD%BF%E7%94%A8-%E5%88%9B%E5%BB%BA%E8%BF%9E%E6%8E%A5"><span class="toc-text">SQL92：使用(+)创建连接</span></a></li></ol></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#3-SQL99%E8%AF%AD%E6%B3%95%E5%AE%9E%E7%8E%B0%E5%A4%9A%E8%A1%A8%E6%9F%A5%E8%AF%A2"><span class="toc-text">3. SQL99语法实现多表查询</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#3-1-%E5%9F%BA%E6%9C%AC%E8%AF%AD%E6%B3%95"><span class="toc-text">3.1 基本语法</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#3-2-%E5%86%85%E8%BF%9E%E6%8E%A5-INNER-JOIN-%E7%9A%84%E5%AE%9E%E7%8E%B0"><span class="toc-text">3.2 内连接(INNER JOIN)的实现</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#3-3-%E5%A4%96%E8%BF%9E%E6%8E%A5-OUTER-JOIN-%E7%9A%84%E5%AE%9E%E7%8E%B0"><span class="toc-text">3.3 外连接(OUTER JOIN)的实现</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#3-3-1-%E5%B7%A6%E5%A4%96%E8%BF%9E%E6%8E%A5-LEFT-OUTER-JOIN"><span class="toc-text">3.3.1 左外连接(LEFT OUTER JOIN)</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#3-3-2-%E5%8F%B3%E5%A4%96%E8%BF%9E%E6%8E%A5-RIGHT-OUTER-JOIN"><span class="toc-text">3.3.2 右外连接(RIGHT OUTER JOIN)</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#3-3-3-%E6%BB%A1%E5%A4%96%E8%BF%9E%E6%8E%A5-FULL-OUTER-JOIN"><span class="toc-text">3.3.3 满外连接(FULL OUTER JOIN)</span></a></li></ol></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#4-UNION%E7%9A%84%E4%BD%BF%E7%94%A8"><span class="toc-text">4. UNION的使用</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#5-7%E7%A7%8DSQL-JOINS%E7%9A%84%E5%AE%9E%E7%8E%B0"><span class="toc-text">5. 7种SQL JOINS的实现</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#5-7-1-%E4%BB%A3%E7%A0%81%E5%AE%9E%E7%8E%B0"><span class="toc-text">5.7.1 代码实现</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#5-7-2-%E8%AF%AD%E6%B3%95%E6%A0%BC%E5%BC%8F%E5%B0%8F%E7%BB%93"><span class="toc-text">5.7.2 语法格式小结</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#6-SQL99%E8%AF%AD%E6%B3%95%E6%96%B0%E7%89%B9%E6%80%A7"><span class="toc-text">6. SQL99语法新特性</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#6-1-%E8%87%AA%E7%84%B6%E8%BF%9E%E6%8E%A5"><span class="toc-text">6.1 自然连接</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#6-2-USING%E8%BF%9E%E6%8E%A5"><span class="toc-text">6.2 USING连接</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#7-%E7%AB%A0%E8%8A%82%E5%B0%8F%E7%BB%93"><span class="toc-text">7. 章节小结</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E9%99%84%E5%BD%95%EF%BC%9A%E5%B8%B8%E7%94%A8%E7%9A%84-SQL-%E6%A0%87%E5%87%86%E6%9C%89%E5%93%AA%E4%BA%9B"><span class="toc-text">附录：常用的 SQL 标准有哪些</span></a></li></ol></li></ol></div></div></div></div></main><footer id="footer" style="background-color: transparent;"><div id="footer-wrap"><div id="ft"><div class="ft-item-1"><div class="t-top"><div class="t-t-l"><p class="ft-t t-l-t">格言🧬</p><div class="bg-ad"><div>再看看那个光点，它就在这里，这是家园，这是我们 —— 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